here are a few of my thoughts. By the way, were you offered $20,000/ac and trying to understand your good fortune? Or your neighbor was and you are trying to understand why you got gipped, or you've read stories of large lease numbers and you are holding out for the big score?.........lol.... anyway,
If they truly can't secure ANY acreage around them, then I agree with you; what's the point, no purpose. However, I doubt that would really be the case, that is, it is likely they have other properties either adjacent or nearby. In general, it is necessary for a company to hold the mineral rights for an entire drilling unit (DU). A drilling unit for unconventional shale is often 640 acres. If they have a majority (I don't know the specifics) of a DU they can force pool "holdouts" to lease and become part of the drilling unit. So that is why ALL acreage is generally needed.
Now regarding why $20,000 an acre. well it can occur but these lease amounts are rare. During the boom in Texas in 2008 some leaf amounts reached $27k if I remember right - but also many leases at that level collapsed/cancelled right along with the economy and were'n't paid.
So it isn't uncommon to see leases for $40 and up. These days, more likely to be $200 and up and if in a hot area then $1,500 - $3,500 per acre isn't unheard of. When to pull the trigger and lease is the question, do you accept $50, 200, 1,000 or wait, granted someday that mineral acreage might have $5,000/ac value. or you got bypassed and a well is near you and making your land/minerals - not needed. It is like playing poker kind of. but in this case you get to go to the Internet and ask questions and you get to drive down the road and ask your neighbor about going rates (what is everyone hearing).
One final point, why a lot of money for a single acre or small parcel? - this can happen if there is a mineral owner that doesn't want to lease and the lessee might offer them more to make the deal. On the other hand if they don't make the deal (at an although higher offer, not astronomical) then the lessee can force pool.
This doesn't matter whether in Texas - it applies universally.